ε-caprolactam may be used as a monomer for producing Nylon-6, that is a raw material for producing fibers, engineering plastics and the like. ε-caprolactam may be produced by a process in which cyclohexanone oxime is evaporated and is then rearranged into ε-caprolactam in a gas phase Beckmann rearrangement reaction. The cyclohexanone oxime has to be evaporated in this process, and a lot of evaporation methods for the cyclohexanone oxime are known.
For example, JP-55-141467-A1 (corresponding to U.S. Pat. No. 4,268,440) discloses a method in which cyclohexanone oxime is evaporated, in the presence of an inert gas, in contact with a fluidized bed of inert solid particles such as silica sand. This method has problems such that a portion of the cyclohexanone oxime is decomposed in the fluidized bed and that much inert gas is needed.
JP-53-37641-A1 (corresponding to U.S. Pat. No. 4,137,263) discloses a method in which cyclohexanone oxime is evaporated in the presence of an inert gas at a super-atmospheric pressure of at least 300 Torr (i.e., at an absolute pressure of at least 1060 Torr) using a falling-film evaporator. This method also has problems such that the cyclohexanone oxime tends to remain in the evaporator and to generate crusts such as tar (or coke) due to the decomposition thereof, which may deteriorate the evaporation efficiency and may plug pipes of the evaporator.